1. Field of the Invention
The present invention relates to a compression-ignition type engine and a combustion method of the same.
2. Description of the Related Art
In a usual compression-ignition type engine, fuel of a mean particle size of about 20 .mu.m to 50 .mu.m or less is injected into the combustion chamber after about 30 degrees before top dead center in the compression stroke. Further, in the special compression-ignition type engine described in Japanese Unexamined Patent Publication (Kokai) No. 60-256523, an intake throttle valve is arranged in the intake passage or an exhaust throttle valve is arranged in the exhaust passage, the fuel injection timing is advanced to 120 degrees before top dead center, and at least one of the intake throttle valve or exhaust throttle valve is closed at the time of a low engine load operation.
However, in an ordinary compression-ignition type engine wherein fuel of an average particle size from 20 .mu.m to 50 .mu.m or less than that is made to be injected after 30 degrees or so before top dead center of the compression stroke, part of the injected fuel is immediately vaporized just when the injection is begun. The succeeding fuel enters into the flame of combustion of the vaporized fuel and thus the injected fuel is successively burned. If the fuel entering into the flame of combustion is made to be successively burned in this way, however, the fuel will be burned in a state of air shortage, so a large amount of unburnt HC or soot will be generated.
In such a compression-ignition type engine, further, the fuel injection is formed in a limited region and therefore the combustion is performed in a limited region in the combustion chamber. If combustion is performed in such a limited region, the local combustion temperature becomes higher than compared with the case where combustion is carried out in the entire interior of the combustion chamber, and accordingly a large amount of NO.sub.x is produced. Further, the smaller the particle size of the injected fuel, the greater the fuel vaporizing immediately upon injection, so the severer the sudden pressure rise caused by the explosive combustion when the ignition delay time passes after the start of the injection and as a result the higher the combustion temperature, so the still greater amount of NO.sub.x which is produced.
Further, in the compression-ignition type engine described in Japanese Unexamined Patent Publication (Kokai) No. 60-256523, the intake throttle valve or the exhaust throttle valve is closed at the time of a low engine load operation so as to increase the amount of the residual burnt gas in the combustion chamber and the heat of that residual burnt gas is used to heat the injected fuel and radicalize the fuel molecules. In this compression-ignition type engine as well, however, the same combustion is performed as in the ordinary compression-ignition type engine other than at the time of a low engine load operation, so at that time a large amount of soot and NO.sub.x are produced.
In this way, so long as the conventional combustion method is used, it is impossible to avoid the production of soot and NO.sub.x and accordingly it is necessary to make fundamental changes to the combustion method in order to prevent the generation of soot and NO.sub.x.